Atmospheric degradation of two short-lived brominated hydrocarbons (CHBr 3 and CH 2 Br 2 )

Abstract

International audienceTwo brominated VSLS, bromoform (CHBr 3) and dibromomethane (CH 2 Br 2), which have natural sources in coastal regions, have the potential to transport reactive bromine into the stratosphere and to contribute to the bromine budget. In order to better evaluate the impact of these two species, chemical schemes for their atmospheric degradation have been developed from a detailed kinetic and mechanistic analysis of all the gas phase reactions involved, specially of the peroxy radicals. The most likely pathways for the reactions of HO 2 with brominated peroxy radicals RO 2 (with R = CH 2 Br, CHBr 2 and CBr 3) have been established using ab initio calculations. The Henry's law constants of the brominated organics products have been also estimated using empirical methods. Using these constants, the less soluble species formed from the brominated VSLS degradation are found to be CBr 2 O, CHBrO, CBr 3 O 2 NO 2 , CHBr 2 O 2 NO 2 , BrO, BrONO 2 and HOBr. In the presence of deep convection, these species could be transported into the TTL (tropical tropopause layer). Then, these data have been implemented in a meteorological/tracer transport model (CATT-BRAMS), including a simplified chemistry of other atmospheric species The full degradation schemes have been run under realistic conditions of " clean " and moderately NO x-polluted atmospheres, which are representative of tropical coastal regions. The influence of the reactions of the RO 2 radicals with HO 2 , CH 3 O 2 and NO 2 on the nature and abundance of the stable intermediate and end-products has been tested. In the case of CHBr 3 degradation, it results that the reactions of RO 2 with NO 2 have no impact, and that the inclusion of the reactions of RO 2 with CH 3 O 2 and with HO 2 (with " new " branching ratios) leads to a slight decrease of the bromine potentially able to reach the TTL. In contrast to CHBr 3 , the CH 2 Br 2 degradation leads to a negligible production of organic species. Finally, for both bromoform and dibromomethane degradation, the effect of a moderate NO x pollution significantly increases the production of the less soluble species and thus approximately doubles the bromine potentially able to reach the TTL. By taking into account the results of these analysis, simplified degradation schemes for CHBr 3 and CH 2 Br 2 are proposed